Combustion engines such as gas turbine engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to this energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustor section. The products of combustion are directed through a turbine section, where they expand and turn a central rotor.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and to achieve desired performance characteristics. One popular combustor design includes a centralized pilot burner (hereinafter referred to as a pilot burner or simply pilot) and several main fuel/air mixing apparatuses, generally referred to in the art as injector nozzles, swirlers, main swirlers or main swirler assemblies, arranged circumferentially around the pilot burner. With this design, a central pilot flame zone and a mixing region are formed. The stream of mixed fuel and air flows out of the mixing region, past the pilot flame zone, and into a main combustion zone of a combustion chamber, where combustion occurs. Energy released during combustion is captured by the downstream components to produce electricity or otherwise do work.
During operation, the pilot burner selectively produces a stable flame that typically is anchored in the pilot flame zone, while the fuel/air mixing apparatuses produce a mixed stream of fuel and air in the above-referenced mixing region. In many designs of conventional pilot burners, the stabilization of the pilot flame is recognized to occur due to a strong, central recirculation zone. An undesired effect of the recirculation zone is the occurrence of regions of higher than average temperatures which result in relatively high levels of undesired combustion products, such as oxides of nitrogen (NOx). Due to concerns and regulations about reduction of undesired emissions from gas turbines, a number of approaches have been taken toward reduction of such emissions while maintaining a suitable efficiency and stable combustion.
Among the approaches taken to address a balanced, low-NOx burner (also referred to by some as a nozzle) is an approach taught in U.S. Pat. No. 5,735,681. That patent teaches a fuel-air nozzle in which a premixed fuel-air mixture is swirled gently by low swirl jets of air introduced tangentially, upstream of the exit port of the fuel-air nozzle. As the fuel/air mixture moves downstream from the fuel nozzle, the flow stream diameter increases, the axial flow velocity decreases and the flame is stated to be positioned where the flame speed matches the flow rate of the fuel-air mixture. This is stated to occur without recirculation which would normally result in an anchoring of flame at a point near the fuel nozzle. The patent further states that “[b]ecause the fuel-air mixture is weakly swirled only at the outside edges of the burn zone, complete burning is possible and NOx emissions are minimized.” U.S. Pat. No. 5,879,148 describes an another approach where a flow balancing insert in introduced into a central passage which surrounds an annular passage of a swirler to produce a stable flame without recirculation.
Despite such approaches in the art, there remains a need to develop an efficient and flexible pilot burner suitable for commercial gas turbine engines, and more particularly for lean premixed gas turbine engines.